Clouds hover over Kennedy Space Center as NASA prepares for the Independence Day launch. Concerns over bad weather postponed two other attempts. To the right, NASA's vehicle assembly building, Pad 39A and 39B -- the launch pad for Discovery -- can be seen across the St. John River from Titusville, Fla.

Clouds hover over Kennedy Space Center as NASA prepares for the Independence Day launch. Concerns over bad weather postponed two other attempts. To the right, NASA's vehicle assembly building, Pad 39A and 39B -- the launch pad for Discovery -- can be seen across the St. John River from Titusville, Fla. (Pete Cosrgrove / AP)